Microwave‐Assisted Carbonyl Diels‐Alder and Carbonyl‐Ene Reactions Supported on Graphite

Abstract

"Dry" carbonyl Diels-Alder and carbonyl-ene reactions with microwaves and reactants supported on graphite were dramatically accelerated with respect to homogeneous conditions both involving classical or microwave heating. Al-though the reactions were carried out in an open reactor, a control of the evaporation of volatile reagents was possible, probably due to their retention or confinement by graphite.Chemistry on solid supports, almost unnatural for organic reactions, has been developed rapidly during the last twenty years [']. The idea of microwave (MW) heating in this reaction type has recently stimulated additional interest[*]. We have recently described "dry" by graphite support MW irradiation and reported a high acceleration of some well-known difficult Diels-Alder reactions. The same process was applied here to two thermal cycloadditions involving carbonyl derivatives as electrophiles, the carbonyl Diel~-Alder[~>~] and the carbonyl-ene r e a~t i o n [~,~] , which are also known to be difficult. Moreover, in this study we tested the possible retentive behavior of graphite towards organic reagentsL7]. To this end, volatile dienes were used in Diels-Alder reactions.At first we treated 2,3-dimethyl-l,3-butadiene (DMB) (1) with ethyl glyoxylate (2) and ethyl mesoxalate (3). With the latter, reactions occurred without a catalyst, giving the expected adduct 5 in 75% yield after only MW irradiation for 20 X 1 min (IP = 30 W; t , , , = 165°C). In the case of conventional heating, it is necessary to heat at 150°C for 4 h in a sealed tube to obtain a satisfactory yield [']. MeWith ethyl glyoxylate, a weaker dienophile than 3, a catalyst (ZnC12) was necessary, as under conventional heating conditions [lO]. Adduct 6 was isolated in 87% yield after MW irradiation for 10 X 1 rnin (IP = 30 W, t , , , = 165°C). As above, conventional heating conditions are more drastic" 'I. Taking glyoxylic acid (4) as dienophile, we obtained the adduct 7 after MW irradiation for 10 X 1 min (IP = 60 W; t , , , = 249°C) in 54% yield. This reaction has not yet been examined by conventional heating, and 7 was previously prepared from S91.The ene reaction or Alder reaction["] is a cycloaddition which requires more energy than the Diels-Alder reaction. However, it can compete with it, in particular for C-alkylated dienes such as DMB. The ene reaction was not detected in the above examples. In 1992, the investigation of a specific activation by MW led us to study the carbonylene reaction in homogeneous medium ['3]. A kinetic approach showed that an athermal (or no thermal) specific effect agrees more with a myth than with reality, as was also true for the esterification of 2,4,6-trimethylbenzoic acid with 2-p r o p a n~l [ '~~] and the Diels-Alder reaction of anthracene with ethyl maleate [14b].In the present work, we take up again some examples of carbonyl-ene reactions under conditions of coupling MW and graphite. The enophile 3 was allowed to react with (-)-P-pinene (8) and 1-decene (9). In homogeneous medium, 8 and 3 react in 6 h in CC14...

Microwave‐Assisted Reactions on Graphite

“…7.2) [30,31]. The reaction of 12 with diethyl mesoxalate gave 14 in 75 % yield after SMW irradiation at low power (30 W) for 20 × 1 min only (Tab.…”

“…The reaction supported on graphite occurred in only 2 × 1 min of MW irradiation with an incident power of 60 W. The adduct 27 was obtained in 67 % isolated yield (Scheme 7.3) [30]. This yield was obtained after 2 h of reaction under CCl 4 reflux.…”

“…When performed in a homogeneous liquid medium at the same temperature but under the action of MW irradiation the reaction gave a similar result. Reaction time was appreciably shortened by use of GS/MW coupling [30]. Thus, irradiation at 60 W for only 10 min led to the ene adduct 25 in 50 % yield (Scheme 7.2).…”

“…7.2 Diels±Alder reactions with 2,3-dimethylbutadiene(12) and isoprene (13) by use of the GS/MW process[27,30,31].…”

Instant modification of graphite nanosheets by the grafting of a styrene oligomer under microwave radiation

“…16,17 Moreover, a carrier such as graphite, which generates a high-temperature gradient because of strong coupling with microwaves, can considerably increase the reaction rate. 18,19 In this work, we report that a St oligomer was successfully and instantly grafted onto the surface of GNs with the help of microwave radiation. The GNs acted both as reagents and as microwave-absorbing support.…”